As shown in FIG. 5, conventionally, a heat insulating structure 51 for building material is manufactured as follows: The surface of a heat insulating material 52 such as fibrous glass wool is covered with a casing material such as an aluminum deposition poly-film 53a. The back face of the heat insulating material is (covered with a casing material of a poly-film 53b. The poly-films 53a, 53b are allowed to adhere to the heat insulating material 52 using a hotmelt 54. Each end of the upper aluminum deposition poly-film 53a is wrapped in each corresponding end of the lower poly-film 53b to form a three-ply structure portion 55, in which poly-films 53a and 53b are allowed to adhere each other by means of thermal fusion.
In the thermal fusion, for example, scored fusing portions f are formed at certain intervals as shown in FIG. 6 or a continuously meandered fusing portion g is formed as shown in FIG. 7, in the longitudinal direction on the portion 55. With this structure, the work especially for integrating the heat insulating structure to, for example, the back face of a wall material does not cause the fused portions to be easily peeled off even if the ply portions 55 of the both ends of the heat insulating structure are pulled in a lateral direction by both hands.
However, in the above-mentioned technology, the three sheets of poly-films 53a and 53b of the portion 55 are fused simultaneously though these form a three-ply structure. This provides combined portions with the same shape and fused portions limited to the scored line parts or the continuously meandered part, which are a part of the face of the portion 55, so that the adhesion of the entire faces of the films are not allowed, exhibiting the problem of weak adhesive force. Also, in the case of thermal fusion in scored portions, for example, a thermal body like a gear is pressed to the portion 55 while it is rotated and the poly-films 53a and 53b are moved in the longitudinal direction, to carry out the thermal fusion continuously. However, if the moving speed for the adhesion is too high, only an insufficient result of fusion is obtained whereas if the moving speed is too low, even the portions around the scored portions melt away, exhibiting the problem that the heating temperature and the heating time are limited. For example, the adhesion speed is limited to about 40 m/minute, which proves that this method lacks flexibility.